14 replies to this topic

[TopCatCustom]

Hi all. I will be welding a full cage into my bare shell, including removing the rear seat back completely and tying the rear legs into the top shock absorber mounts. I will of course be putting some form of rear bulkhead/firewall back in to seperate the fuel tank and battery from the inside of the car, with aluminium.

The car will be used for club rallying etc, and I want to keep a good compromise between keeping it lightweight, strong and rigid. I want to cut out and box the rear shock towers to make them much stronger (and provision for coilovers should I ever go down that route) which should be easy enough, and I have thought about tying the read subframe mounts to the new shock towers, with large triangular folded pieces of thin steel to stop any flex, and "swiss cheesed" to save weight obviously.

Once I have done all this I will weld a thin steel strip on edge down the inside of the rear quarters where the seat back would be and pop rivet a folded and swaged ally sheet there for the new rear firewall. The other advantage is that the existing rear seat back does not fill the full width so it won't suffice as a firewall without modification anyway, and I can fit it around the rear legs of the roll cage neatly too.

Does it sound like a plan?! All constructive thoughts etc welcome!

Thanks,
TC

[Dan]

One point would be that a fuel fire is about 300 degrees hotter than the melting point of aluminium!

[TopCatCustom]

I think there's more chance of the petrol in the boot exploding and ripping the boot lid and seat away than slowly burning hot enough to melt it! There are thousands of cars with aluminium panels etc that would melt in theory!

[Cooperman]

That does sound like a good spec for club special-stage rallying. In themselves the rear damper towers are quite strong, but a bit of extra strength can only be a good thing.
In fact, the basic Mini shell is quite good in terms of torsional and longitudinal rigidity and I can't remember ever seeing the rear sub-frame rear mountings moving up under repeated shock loading, so the weight penalty of adding triangular webs to support the baco of the frame may be really necessary.
An aluminium rear fire-wall willl be acceptable to scrutineers and with a strip welded on to fill the original gap you'll be fine. The cage will replace any loss of shear strength which was originally provided by the rear seat panel.
Presumably you'll have a competition fuel tank with foam filling, so fire should not be such an issue.
Will you be putting the battery inside the car in a fully sealed battery box?
If you look a t the Mini shell, it's a really good box-shape which is why it is basically so strong (so long as it ain't rusty!). I've rolled a few times in Minis on various rallies and usually the cars were driveable afterwards with damage mainly to body panels. The last time all I needed was a roof panel and a bit of painting. The cage just took it all with no problem.
I'd love to see some photos of your project.

[TopCatCustom]

Hi again Cooperman, thanks very much for your input as always. Would you suggest leaving the shock towers as standard, and just welding a stronger top on and still tying the rear cage legs into them? The rear will be so light I hope the subframe mounts will not flex and if you think they are ok in a standard car then they should be fine for me, I'm aiming to get the car into the mid 500kg area, which may be optimistic I know but I'm not constrained by using all production parts like yourself with your original "S".

I do have a build thread which is here http://www.theminifo...howtopic=172080

[Cooperman]

It doesn't do any harm to strengthen the rear damper towers, although if you fit decent bump stops on the rear it is not an issue. If you can tie in the cage to the tower top rather than the adjacent inner wheel arch, so much the better.
The car is going to be very light - whether you'll get down to 500 kg is another matter. I weighed mine in full rally trim, with perspex windows, no soundproofing, but with full FIA cage, extra instrumentation, 2 spare wheels, 1/2 full with fuel, etc, it weighed in at 718 kg.
For rallying the comp quality rubber cones will probably prove best in performance terms as they are truly 'variable rate' springs.
The big problem with the Mini against modern cars is its lack of traction on bumpy roads due to insufficient suspension travel. It helps if you raise the ride height a bit and don't set the damping rate too hard to allow the suspension to work over the max travel possible. Tyres are also a problem as you obviously don't want 13" wheels and with 12" tyre choice is limited compared to modern cars like the Rover 214, Pug 106, etc, which have 14" or 15" wheels with lots of tyre types and compounds available.

[TopCatCustom]

I was going to fit standard new rubber cones, are the comp ones stiffer or something? I was also going to use hi-los to get some more height, and Gaz adjustables as I like them. 10" wheels which I believe there is a reasonable choice of tyres for, I know it wont be competitive against newer cars but on smoother ground and tight corners hopefully the weight and size advantage will prevail! I dont think I will be carrying spare wheels to start with, and my dash etc will be minimal.

[bmcecosse]

The weight saving by cutting out the rear seat back etc will be minimal - and the bodyshell will lose a LOT of it's strength. My advice - don't do it! Concentrate your efforts elsewhere! Removing the rear subby will save a LOT more weight - just use a front beam instead.

[TopCatCustom]

It will be replaced with aluminium- and the edges will be steel and welded a lot more than the original seat, with the aluminium pop riveted every 100mm all the way round so should be pretty stiff- especially given that a full cage is going to be tyed into the shell at every available point. The weight saving should be 6kgs. I'm going to be looking at lightening the rear subframe too, and making some tubular radius arms but I want to keep the rubber suspension which is why I need to keep the subframe.

[Cooperman]

With the multi-point welded in FIA cage, removal of the back seat panel and its replacement with an aluminium panel will not have any effect on the shell's strength.
Personally I wouldn't use a beam rear axle on a rally car. OK for racing, but rallying is so different and a rear sub-frame is really necessary.
The big problem with 10" wheels these days is the unavailability of 10" forest tyres. I still have some 10" Colway 'Nora' pattern gravel ones, but they are now unavailable and you'll end up with a 12" wheel for gravel/forests or the narrow 'Hakka pattern' 145/80x10.
The standard original spec cones are fine and Hi-lo's with GAZ dampers will be just right. Get the hydrolastic rear bump stops for the rear arms and they will take out the worst of the really big bumps. For forests it would be good to raise the car around 3/4" over standard ride height, with it back to standard for tarmac. Never run a Mini below standard ride height for rallying.
Also fit a battery box shield under the battery box, bolted to front and rear of the sub-frame. I use 3 mm thick aluminium sheet with a 1" x 1" aluminium angle rivetted along the inner edge.
If I were you I would keep away from the MPI block and just go for an A+ one. You will need an oil cooler and the MPI is not as easy to fit one to as an A or A+ block.
With regard to weight, I think you may get down to around 625 kg if you lighten absolutely everything, remove trhe rear side bins, use aluminium door skins, fibreglass or carbon-fibre bonnet and boot lid, etc. Just cut away anything which is not structural like the lip on the rear seat base panel, front parcel shelf, all soundproofing, replace rear parcel shelf with an aluminium panel picking up on around a 1.5" lip left from the original. I have even seen an aluminium roof panel rivetted onto a 2" wide lip left on the steel roof, but I don't think you'll find one of these as the one I saw was back in the 60's on a race car.
I've always believed the front sub-frame is over-engineered. You could drill lightening holes in the big almost-vertical webs, narrow down one of the tranverse webs across the front as the sump guard will put that strength back in when bolted in with 4 big bolts across the front and 2 each side under the lower arm mounting area.
With a full cage and door bars, plus perspex windows, most of the inner door skin can be cut out which saves a lot. Removal of the window winder mechanism is also good. Just make up a little sliding window in the middle of the main perspex window.
As they say, in weight saving, 'every little helps'.
I look forward to seeing your progress. I'll see about a shell this weekend when I take out MPI for painting (assuming we are not snowed in!).

Peter

[Ivor Badger]

It will be replaced with aluminium- and the edges will be steel and welded a lot more than the original seat, with the aluminium pop riveted every 100mm all the way round so should be pretty stiff- especially given that a full cage is going to be tyed into the shell at every available point. The weight saving should be 6kgs. I'm going to be looking at lightening the rear subframe too, and making some tubular radius arms but I want to keep the rubber suspension which is why I need to keep the subframe.

Thw weight saving will be no where near 6kgs. Pop rivets every 100mm, that won't add much strength. I have to check the maths, but proper field rivets around 15mm or closer would be nearer the mark. Having done it once mistakenly, I never did it again. I ended up with a lot of work and it probably weighed the same when I finished, but the shell lost a lot of strength. You could see the stress marks on the aluminium.

Follow BMC Eccosse's advice and throw away the subframe. If the rear beam system breaks because of the loads, the new rear bulkhead will have failed first.

For club rallying? find a lighter navigator.

[TopCatCustom]

I roughly calculated that the existing steel there weighs around 9.5kg, the same size peice of aluminium weighs around 3.3kg, to me that makes a 6.2kg difference, add some thin steel strip around the edge and its not far off 6kg?

Rivets every 15mm?? I'm not building a high altitude fighter! I'm just going by what I've seen in at least 3 mk1 and 2 escorts that have been rally prepped, and I'd have thought that a cage with a X low down bracing the rear shocks/legs to main hoop feet, and another X between the main hoop top corners and rear shock/rear legs will actually stiffen it considerably. I had coilovers on my last mini and they seemed too stiff for the light weight of the mini, which is why I'm put off by them again. If it was a track car I'd be working this differently.

[Wil_h]

I would double check the regs about what you can and can't remove. For hillclimbing all the shell mods you propose are not allowd.

[TopCatCustom]

Open regs for me, I had planned to build a strict mk1 S replica but that went out the window when I got my clean later shell and realised how much money original works bits were going for!

[TopCatCustom]

Hmm, I'm now considering coilovers on the rear... I'm not sure if they would give much benefit on the front as if they give much more suspension travel then could it cause problems with the driveshafts popping out of the joints on rough stages where they would allow a lot of movement?

On the rear I think there is less to lose and if they are soft enough (because it will be lighter than an everyday mini loaded up with junk) then hopefully there wouldn't be any disadvantages...

The only downside I can think of is that they normally just fix on with the bolt that sticks out for the damper- so a little (5/8"?) bolt with a single shear plane holding the whole car up???